One area of tire technology that has been developed during the last few decades is the concept of a pneumatic tire that is capable of operating in an under-inflated or non-inflated condition. In such types of pneumatic tires, which are often referred to in the industry as “run-flat” tires, it is generally desired for an under-inflated or non-inflated tire to support a vehicle during operation for a predetermined minimum number of miles and at speeds up to a predetermined maximum speed of operation. The advantages of such a tire in safety and convenience are well documented.
One example of a run-flat tire construction that has been developed over the years is the band-reinforced radial tire, which was invented by the inventor of the present application. Typically, a banded run-flat tire is a pneumatic radial tire having a casing with a crown and sidewalls extending from the crown on either side to annular beads which, in a conventional way, are used to mount the tire in a sealed relationship on the rim of a wheel. The band element is embedded in the crown of the tire underlying the tread during the manufacture of the tire while the tire is in a green or uncured state. In some cases, the band element may be a thin structural ring of high-strength steel or a fiber/epoxy composite, such as is disclosed in U.S. Pat. Nos. 4,111,249; 4,318,434; 4,456,048; 4,734,144; 5,879,484; 6,112,791; 6,117,258; 6,148,885; 6,321,808; 6,363,986; 6,405,773; 6,420,005; 6,436,215; 6,439,288; 6,460,586; 6,470,937 and 6,598,634, for example. In other cases, the band element may take the form of a helical structure or coiled member, such as is disclosed in U.S. Pat. Nos. 4,673,014; 4,708,186 and 4,794,966, for example.
Another example of a run-flat tire construction involves the use of a finished or cured tire of an otherwise standard construction. In this type of run-flat tire construction, a pre-curved, helical or coil-like structural element is formed. The pre-curved coil-like structure is then inserted into the inner cavity of the finished tire in a suitable manner, such as by winding up the helix to reduce the outside diameter thereof or by separating an end of the coil-like structure and feeding the structure into the inner cavity due to relative rotation between the structure and the tire. Such a run-flat tire construction is, for example, disclosed in U.S. Pat. Nos. 4,428,411 and 4,459,167.
With reference to such known constructions, FIG. 13 of the present application graphically illustrates relative stress levels that would be expected to occur in known run-flat tire constructions that rely upon the use of pre-curved bands. The general outline of a pre-curved band is indicated by reference number 50 in FIG. 13. Pre-curved band 50 is shown undergoing the deflection expected during use, with a ground contact patch 52 being formed along a road or other surface 54. Additionally, FIG. 13 illustrates relative stress levels that would be expected to be experienced by the pre-curved band during such use. Inner surface stresses are represented by line 56 and outer surface stresses are represented by line 58 with compression being represented by line portions disposed radially-inwardly from band 50 and tension being represented by line portions disposed radially-outwardly from band 50.
It has been observed that conventional pre-curved bands operating within an under-pressurized or non-pressurized tire will normally have the capability (i.e., durability) to exceed a 100 mile performance target. However, it has also been recognized that such known pre-curved bands may provide less than the desired level of performance during normal, pressurized operation of the tire. It will be appreciated that during normal, pressurized operation, a pre-curved band could be subjected to cyclic flexing and corresponding cyclic variation in stresses many tens of millions of times to reach an 80,000 mile performance target.
It is believed that one reason for the less than optimal level of performance of known pre-curved bands during pressurized operation involves the relative variation in stresses to which known bands are subjected during use. That is, it has been determined that the curvature of pre-curved bands forward and aft of the ground contact area results in relatively low stresses being included in these fore and aft areas. In FIG. 13, pre-curved band 50 would rotate in the direction of arrow RT with areas forward of ground contact patch 52 being indicated by reference characters FWD and areas aft of ground contact patch 52 being indicated by reference characters AFT. It has also been determined, however, that while a given portion of a pre-curved band is disposed within the ground contact area, the pre-curved band is flexed from the initially pre-curved condition into an approximately flat state. During this approximately flat condition, the stresses within the pre-curved band may be substantially higher than the relatively low stresses in the fore and aft areas. What's more, if a positive obstacle is encountered by the tire, the resulting stresses in the pre-curved band at the ground contact area will be further increased.
Furthermore, it is well understood that tension/compression-type cyclic loading can accelerate the decrease in performance of load bearing members, such as may be due to material fatigue, for example. It will be recognized from FIG. 13 that in areas forward of ground contact patch 52, stresses along inner surface 56 of pre-curved band 50 are due to compression and that stresses along outer surface 58 are due to tension. As pre-curved band 50 approaches ground contact patch 52, however, an area of deflection 60 is reached at which the load conditions are reversed and stresses along inner surface 56 are due to tension with stresses along outer surface 58 being due to compression. As pre-curved band 50 exits ground contact patch 52, another area if deflection 62 is reached in which the load conditions are again reversed such that stresses along the point on inner surface 56 are again due to compression and stresses at the corresponding point on the outer surface are again due to tension. As such, known pre-curved bands are typically exposed to a cyclic load condition during each rotation of tire and it is believed that such cyclic loading may contribute to any decreased performance of such pre-curved bands.
Although known run-flat pneumatic tire constructions generally operate satisfactorily, the desire remains to increase performance (e.g., run-flat mileage, pressurized mileage and maximum speed of run-flat operation) and reduce manufacturing costs. As such, the subject concept seeks to provide these and other benefits and/or improvements over known run-flat pneumatic tire constructions.